Saint Stephen's day (Szent István)
The 20th of August is probably the most important national holiday in Hungary - and is celebrated accordingly, especially in the capital.
The Pearl of the East
Budapest is breathtakingly beautiful - especially when you are lucky enough to sit on Fisherman's Bastion close to Matthias Church while the sun is slowly setting behind the white towers and the whole Danube promenade - including the impressive House of Parliament - is dipped in the last rays of light.
And for this view you are certainly ready to bear the masses of tourists and visitors who decided to stay at the very same place to watch the fireworks honouring Saint Stephen.
Speedrun through Budapest
Budapest is the capital of Hungary and it's biggest city by far. The Danube seperates the city in two parts, Buda (on the left side of the Danube) with the old castle borough and Pest, with St.-Stephen's Basilika and the House of Parliament.
On the 20th of August, National Holiday of the Founding of State, Saint Stephen's day and Feast of the New Bread (because on the 20th of August the first bread made from the newly harvested corn is made) a lot of sights in Budapest are free of fee to visit and opened a lot longer than usually - like the Hungarian crown in the House of Parliament or the Fishermen's Bastion, where you can stay until long after nightfall to marvel at the half-hour long firework starting at 21:00.
With Metro, bus and tram nearly all of these sights can be visited every minute - a daily ticket for the whole public transport network of Budapest is available for around 5 €.
Because of the masses of tourists and visitors who have travelled to Budapest just for this day I wouldn't recommend taking time for one or two specific aims to look into more closely, but it is perfect for a first trip through this wonderful city.